This invention relates to a multipurpose liquid crystal display suitable for use as a reflective display and as an overhead projector imaging panel.
Reflective liquid crystal displays, comprising a liquid crystal-based electrooptically active element placed in front of a reflector, are well known in the art. The electrooptically active element transitions between a first and a second optical state in response to an input such as an electrical signal. Light incident on the display does or does not reach the reflector depending on the optical state and accordingly is or is not reflected by the reflector, thereby providing different viewing states to the observer. Reflective liquid crystal displays find many applications, one of the most popular of which is as a computer screen in portable or "laptop" computers, on account of their light weight and low power consumption.
Recently, liquid crystal overhead projection imaging panels have become popular for displaying information, such as data, text or graphics, stored in a computer. The panel is connected to the computer, and output from the computer which would normally be directed to the computer screen is alternatively or simultaneously directed to the panel. The panel is placed directly on the overhead projector where a conventional overhead transparency would be placed, so that the computer generated images are projected by the projector onto a screen. For example, the panel can be transparent except for the dark word "HELLO" formed thereon, in which case the word "HELLO" is projected onto the screen. Such a panel provides a convenient alternative to the conventional method of printing the information onto paper, making a transparency from the paper copy, and placing the transparency on the overhead projector to project the image. Further, while the conventional method requires substantial advance preparation to generate a limited number of transparencies for projection, the panel permits the on-the-spot generation of an indefinite number of images for projection.
A preferred type of liquid crystal display employs encapsulated liquid crystal material, in which liquid crystals are encapsulated or dispersed in a matrix (or containment medium) which can be a polymer. When a voltage corresponding to a sufficiently strong electric field is applied across the encapsulated liquid crystal material (the "field-on" condition), the alignment of the liquid crystals is re-oriented in accordance with the field, so that incident light is transmitted. Conversely, in the absence of such a voltage (the "field-off" condition) the alignment of the liquid crystals is random and/or influenced by the liquid crystal-matrix interface, so that the liquid crystal material scatters incident light. The applied voltage at which the liquid crystal material begins to change from its field-off condition to its field-on condition is called the threshold voltage.
The nature of the reflector has an important effect on viewing quality. In the field-off condition, the display appears bright or paper-white, due to scattering of light towards the viewer. If a specular (mirror-like) reflector is used, good darkness can be obtained in the field-on condition, but images of objects in the room may be reflected to the viewer, producing glare, especially where there are strong point light sources such as light non-diffuse light fixtures. Alternatively, diffuse reflectors have been proposed. While reducing the aforementioned disadvantage, they have the limitation of also reducing contrast. An improved display is obtained where the reflector is a retroreflector, because a retroreflector reflects light towards the source, avoiding the glare problem associated with specular reflectors, but at the same time producing high contrast, unlike displays having diffuse reflectors. The use of retroreflectors in liquid crystal displays is disclosed in Meyerhofer, U.S. Pat. No. 3,905,682 (1975).
Thorn EMI, EP 421810 (1991), discloses a liquid crystal display having a directional reflector comprising a faceted sheet of refracting material and a layer of optically absorbing material separated from the refracting material by a substance whose refractive index is lower than that of the refracting material, for example air.
Cromack, U.S. Pat. No. 4,726,662 (1988), discloses a liquid crystal display including a prismatic lens system or a prismatic reflective system. The prismatic reflective system embodiment comprises a plurality of right-angle reflectors and light absorbing targets. Depending on whether the incident light is inside or outside the viewing cone, it is either reflected towards the targets (and absorbed thereby) or towards the viewer.
Fergason et al., U.S. Pat. No. 4,732,456 (1988), discloses a display with enhanced contrast in which a lens or reflector means is used to direct light transmitted by the display onto a light absorbing target.
Fergason, U.S. Pat. No. 4,613,207 (1986), discloses a liquid crystal projector in which an image or the characteristic of an image are created on a liquid crystal display and projected using an optical projection arrangement.
Wiley, U.S. Pat. No. 4,693,560 (1987), discloses a double layer display comprising a front and a rear liquid crystal cell, a transflector which partially reflects and partially transmits incident light disposed between the two cells, and a backlight source behind the rear cell. Under dark ambient conditions, the display operates as a backlit transmissive display. Under bright ambient conditions, the display operates as a reflective display comprising the front cell and the transflector.
Dalisa et al., U.S. Pat. No. 4,991,940 (1991), discloses a reflective liquid crystal display in which an off-set gain reflector is used to separate the directions of the gain and glare light.